1. Field of the Invention
This invention relates to a microwave amplifier and more particularly to a broad-band, frequency tunable microwave amplifier having a hybrid idler circuit and a semiconductor diode operating in the TRAPATT mode.
2. Description of the Prior Art
Microwave amplifiers having high power and efficiency and bandwidths greater than 10 percent of the central frequency of operation are desirable, for example, in phased-array radar systems, and communications systems. Prior art amplifiers utilizing TRAPATT diodes have generally required extensive circuit adjustments during fabrication to achieve a broad-band amplifier by making the "instantaneous bandwidths" as wide as possible without sacrificing gain or efficiency. The "instantaneous bandwidth" of these devices as known in this art is the range of input frequencies over which a signal can be amplified at a particular instant of time without any mechanical or electrical adjustments or other tuning.
As is well known, the wave form of a signal generated or amplified by a semiconductor diode operating in the TRAPATT mode is rich in harmonic content. It is generally recognized that a successful TRAPATT amplifier design must provide certain load impedances at the fundamental trapped plasma frequency and at least the second and third harmonic thereof. The impedance loading conditions for such an amplifier are achieved in the prior art typically by using a separate tuned circuit for each harmonically related frequency. For example, each circuit is tuned, i.e. adjusted, to have a desired impedance condition, by placing an inductance element in series with a lumped capacitance element as disclosed in U.S. Pat. No. 3,909,740, issued on Sept. 30, 1975, or by placing tuning stubs in a transmission line as disclosed in U.S. Pat. No. 3,848,196, issued on Nov. 12, 1974. In U.S. Pat. No. 3,868,588, issued on Feb. 25, 1975, one of the tuned circuits is eliminated by providing a semiconductor element which is self-resonant at one of the desired frequencies, thereby precluding the need for one of the tuned circuits.
None of the above-mentioned devices, nor, as far as is known, any other prior art amplifier using TRAPATT diodes, provides for an increase in bandwidth by frequency tuning the device. In order to frequency tune such amplifier devices, the conditions of impedance at each of the harmonic frequencies must be maintained throughout the desired tunable bandwidth. These prior art devices are initially adjusted for a set of impedance conditions, depending upon the preselected frequency at which power is to be extracted, and rely upon a decrease in the Q factor of the circuit to increase the instantaneous bandwidth for broad-band applications. Any adjustment in the impedance condition at the fundamental frequency or any of the related harmonic frequencies requires a readjustment in the other impedance conditions to operate the amplifier. Disadvantageously, therefore, the frequency of such devices is not tunable by simply adjusting the resonant impedance conditions at the fundamental frequency. Such devices would require in addition to a simple resonant impedance adjustment further adjustments in the other circuits.